Multi experimental and computational studies for DNA and HSA interaction of new nano-scale ultrasound-assisted synthesized Pd(II) complex as a potent anticancer drug

Article abstract of DOI:10.1016/j.molliq.2018.05.077

As for daily increasing mortality rate in world due to the growth of cancer causing agents, design and synthesis of new compounds with anticancer potential benefits is one of the most important challenges for researchers. In the present work, we synthesiz

Full text of DOI:10.1016/j.molliq.2018.05.077

Journal of Molecular Liquids 264 (2018) 386–397
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Multi experimental and computational studies for DNA and HSA
interaction of new nano-scale ultrasound-assisted synthesized Pd(II)
complex as a potent anticancer drug
a
a,
a,
a
b
Monireh Dehkhodaei , Mehdi Sahihi ⁎, Hadi Amiri Rudbari ⁎, Mahnaz Ariaeefar , Sajjad Gharaghani ,
c
d
a
Reza Azadbakht , Salman Taheri , Abolghasem Abbasi Kajani
Department of Chemistry, University of Isfahan, Isfahan 81746-73441, Iran
Laboratory of Bioinformatics and Drug Design (LBD), Institute of Biochemistry and Biophysics, University of Tehran, Tehran, Iran
Department of Chemistry, Payame Noor University, P.O. Box 19395-3697, Iran
Chemistry and Chemical Engineering Research Center of Iran, Tehran, Iran
a
b
c
d
a r t i c l e i n f o
a b s t r a c t
Article history:
As for daily increasing mortality rate in world due to the growth of cancer causing agents, design and synthesis of
new compounds with anticancer potential benefits is one of the most important challenges for researchers. In the
present work, we synthesized a new Schiff base Pd(II) complex in bulk-scale and also in nano-scales by
Sonochemical method. The structure of synthesized complex was determined by single crystal X-ray diffraction
technique. Then the cell viability percent of HeLa cancer cells was studied by MTT assay. The results confirmed
that reducing the size has salient effect in annihilation of cancer cells. Also, nano-scale complex reached to
IC50 in 10 μM of concentration. Binding ability of the nano- and bulk-scale Pd(II) Schiff base complex with calf
thymus DNA and human serum albumin was investigated using combination of experimental (fluorescence, cir-
cular dichroism (CD) and viscosity) and computational (molecular docking, molecular dynamics simulation and
QM/MM) methods. The estimated binding constants for the complex in both of bulk- and nano-scales showed
that the nano-scale complex binds more tightly to DNA than its bulk-scale form. This finding is in good agreement
with MTT assay results. Molecular docking studies revealed that Pd(II) complex binds to the minor groove and IB
binding site of DNA and HSA, respectively. Also, MD simulation studies showed that complexation with the Pd(II)
complex changes the structure of HSA with compared to free protein. Finally, the ONIOM results indicated that
the structural parameters of the compound changed along with binding to DNA and HSA, indicating the strong
interaction between the compound and these biomacromolecules. The values of binding constants depend on
the extent of the resultant changes.
Received 29 December 2017
Received in revised form 25 April 2018
Accepted 18 May 2018
Available online 19 May 2018
Keywords:
Sonochemistry
Nano complex
Fluorescence enhancement
MD simulation
Molecular docking
QM/MM
©
2018 Elsevier B.V. All rights reserved.
1
. Introduction
the interaction between HSA and small drug molecules is very impor-
tant for understanding the mystery of life and the interaction mecha-
nisms of drugs. The results of investigation show that more than 90%
plasma protein can be combined with metal drugs in form of covalent
bonds. Studying on the metal base drugs is one of the basic area in
bioinorganic chemistry field. The researchers in this field try to discover
compounds with more impressive properties and less side effects from
cis-platin that is known as appropriate anti-cancer drug. One of the ap-
proaches is using the synergic effect between metals and biological li-
gands. Schiff bases are the class of inorganic ligands that their
biological activities have been proven, previously [14–16]. On the
other hand, the simplistic synthesis and wide range of applications of
Schiff base ligands have attracted a lot of interests. These compounds
play an important role in development of coordination chemistry re-
lated to catalysis, organic synthesis [17,18], antitumor, antimicrobial
and cytotoxic activity [19–22]. In recent years, remarkable attention
Platinum-containing drugs such as cis-platin, carboplatin and
oxaliplatin show their anti-cancer activity based on in-vivo interaction
with the DNA of the cancerous cells. Studying on the interactions be-
tween DNA and the other transition metal complexes is key step to de-
velop new metal based therapeutics [1]. Furthermore, some studies
have investigated binding of the anti-cancer drugs to the Human
Serum Albumin (HSA), as the most abundant plasma carrier protein in
the blood stream [2,3]. The binding ability of a drug to the non-
specific transporter protein HSA is an important research field in chem-
istry, biochemistry and clinical medicine [4–13]. Hence, investigation of
⁎
Corresponding authors.
E-mail addresses: m.sahihi@chem.ui.ac.ir, (M. Sahihi), hamiri1358@gmail.com
(H.A. Rudbari).
https://doi.org/10.1016/j.molliq.2018.05.077
167-7322/© 2018 Elsevier B.V. All rights reserved.
0

M. Dehkhodaei et al. / Journal of Molecular Liquids 264 (2018) 386–397
387
has been paid to biological applications of the metal complexes of Schiff
bases due to their stability, biocompatibility and biological activity
180 W for 30 min and was continuously stirred. The suspension was
centrifuged at 5000 g for 15 min, and the supernatant was withdrawn
and filtered through a 0.2 mm pore-size syringe filter [36].
[23,24]. Their biological activity is due to the existence of imine group
in their chemical structure [25–27]. Furthermore, complexation of Schiff
base ligands with transition metal ions, enhances their biological activ-
ities [28–30]. Some palladium complexes had shown promising results
as antimalarial and anti-cancer agents in previous investigations
2.3. Single crystal diffraction studies
[
31,32]. Also, due to remarkable similarity between Pd(II) and Pt(II)
X-ray data for PdL
a Bruker APEX II CCD area detector diffractometer using Mo K
(k = 0.71073 Å). Data collections, cell refinements, data reductions and
absorption corrections were performed using multiscan methods with
Bruker software [37–39]. The structure was solved by direct methods
using SIR2004 [40]. The non hydrogen atoms were refined anisotropi-
2
complex was collected at room temperature with
radiation
complexes [33]; and their less toxicity and higher activity in compared
to platinum complexes; they are regarded as potential anticancer sub-
stances and have better anticancer activity than their Pt(II) analogues
in some cases [34].
Due to the importance and significant growth of nanotechnology in
all sciences, its role in the field of biology and drug design cannot be ig-
nored. Previous studies showed that the interaction between nanopar-
ticles and biomolecules can change the biomolecule conformation and
perturb the normal protein function which can be lead to unexpected
biological reactions and toxicity. Therefore, investigation of these
types of interactions could not be ignored [35].
α
2
cally by the full matrix least squares method on F using SHELXL [41].
All hydrogen atoms were added at ideal positions and constrained to
ride on their parent atoms. Molecular graphics were prepared with
2
the Olex program [42]. Crystallographic data for complex are listed in
Table S1. Selected bond distances and angles are summarized in
Table S2.
2
Herein, a new Schiff base Pd(II) complex (PdL ) was synthesized and
its molecular structure was determined by single crystal X-ray diffrac-
tion technique. The cell viability percent of HeLa cancer cells was first
studied by MTT assay. In order to increase the colloidal stability and
suitability for the biomedical applications, the nano-scale compound
was also synthesized using ultrasound-assisted method and used for
in vitro studies. Finally, binding ability of the nano- and bulk-scale
2.4. Cell viability assay
MTT assay was used to investigate the anticancer potential of the
bulk- and nano-scale PdL on human cervical cancer cell line (HeLa) ac-
2
cording to the previously reported procedure [43]. The cells were first
PdL
2
with calf thymus DNA (CT-DNA) and HSA was investigated using
cultured in RPMI-1640 medium supplemented with 10% FBS and 1% an-
combination of experimental (fluorescence, circular dichroism (CD)
and viscosity) and computational (molecular docking, molecular dy-
namics simulation and qm/mm) methods.
2
tibiotics solution and were maintained in a humidified 5% CO incubator
at 37 °C. When the cell confluence was reached to 70%, the cells were
harvested and seeded on 96-well plates at a density of 104 cells per
well containing 200 μl medium and incubated overnight at the same
conditions. After exposure to different concentrations of each com-
pound for 48 h, the medium was removed and 100 μl MTT solution
2
. Experimental section
−1
2
.1. Materials sources
(0.5 mg·ml in media) was added into each well and the plates were
incubated again at 37 °C for 4 h. The medium was then completely re-
moved and the formazan crystals were dissolved in 150 μl DMSO and
the absorbance was measured at 570 nm. Each experiment was con-
ducted in triplicate and the results were presented as the mean values
obtained from three independent experiments. The cell viability was
determined as ratio of absorbance values from each treatment and the
control.
All of the used chemicals for synthesis of the complex including
-amino-propane (Isopropylamine), 2-hydroxybenzaldehyde
2
(
salicylaldehyde), triethylamine and Palladium chloride (PdCl2) were
purchased from Merck Co. and were used without further purification.
All of the used salts for buffer preparation were analytical grade and
were dissolved in double distilled water. All of the solutions were
used freshly after preparation. Also, HSA, CT-DNA, RPMI-1640 medium,
Fetal bovine serum (FBS), Dimethyl sulfoxide (DMSO), antibiotics
(
2
penicillin-streptomycin) solution, and 3-(4,5-Dimethylthiazol-2-yl)-
,5-Diphenyltetrazolium Bromide (MTT) were obtained from Sigma-
Aldrich.
2.5. Preparation of DNA, HSA, bulk- and nano-scale PdL
binding experiments
2
stock solutions for
The stock solution of CT-DNA was prepared in 50 mM Tris buffer at
pH = 7.5 using double-distilled deionized water and was stored at 4
2
2
.2. Synthesis of the bulk- and nano-scale Pd(II) Schiff base complex (PdL )
°C. The CT-DNA concentration per nucleotide was determined using ab-
A methanolic solution (30 ml) of Isopropylamine (IUPAC name: 2-
amino-propane) (5 mmol) was added slowly to 30 ml of a methanolic
stirred solution of salicylaldehyde (IUPAC name: 2-
hydroxybenzaldehyde) (5 mmol) in ambient temperature. The colour
immediately changed to yellow and the mixture was then stirred for
sorption intensity at 260 nm after adequate dilution with the buffer and
−1
−1
using the reported molar absorptivity of 6600 M ·cm [44]. Purity of
CT-DNA solution was confirmed by ratio of UV absorbance at 260 and
280 nm (A260/A280 = 1.9), indicating that CT-DNA is free from protein
impurity [45]. Also, a stock solution of HSA was prepared by dissolving
the desired amount of HSA in 50 mM phosphate buffer (pH = 7). The
HSA stock solution was stored at 4 °C in the dark and was used within
2 h. HSA concentration was determined by UV–Vis spectrophotometry
2
(
h. Then a solution of triethylamine (7 mmol) in absolute methanol
10 ml) was added dropwise to ligand solution. The mixture was stirred
for 15 min again. Then, a solution of appropriate PdCl (2.5 mmol) in
2
−
1
−1
methanol (30 ml) was added to the mixture, gently, for synthesis of
the complex in bulk-scale. The solution was refluxed overnight to pro-
ceed completely. The resulting yellow powder was isolated by filtration
and washed by dry methanol several times. Appropriate single crystals
for X-ray crystallography were obtained directly from the reaction mix-
ture. In order to prepare the complex in nano scale, a saturated solution
was prepared by adding excess powder to 10 ml of DMF at room tem-
perature. The solution was pumped from a small orifice into 100 ml of
the antisolvent (water), which was placed in an ultrasonic bath at
using the molar absorption coefficient 35,700 M ·cm at 278 nm
[46]. For preparation of PdL solution in bulk-scale, appropriate
2
amounts of obtained complex powder was dissolved in DMSO as co-
solvent, and then diluted with corresponding buffer to the required con-
centration for all experiments. The volume of co-solvent never
exceeded 5% (v/v), so the effect of DMSO is negligible. Also, the appro-
priate amount of the nano-scale PdL powder was dispersed in Tris
2
and phosphate buffer for DNA and HSA binding experiments,
respectively.

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M. Dehkhodaei et al. / Journal of Molecular Liquids 264 (2018) 386–397
2
.6. Fluorescence spectroscopy measurements
of the .pdb files were deleted, missing hydrogen atoms and Gasteiger
charges were added. Flexible ligand docking was carried out by
AutoDock 4.2.5.1 molecular docking program using the implemented
empirical free energy function and the Lamarckian Genetic Algorithm
[48]. The Gasteiger charges were added to prepare the macromolecule
input file for docking and the AutoGrid was used to calculate the grids.
Fluorescence experiments were carried out using quartz cuvette
with 1 cm optical path length and the excitation and emission slits
were set at 5 and 10 nm, respectively. For investigation of HSA interac-
tion with PdL
the quartz cell and various amounts of PdL
for both of bulk- and nano-scale). The fluorescence intensities were
2
by this technique, 2 ml of HSA solution was placed into
2
were added to the cell
2
For docking of the PdL complex to HSA and DNA, a blind docking
(
with 126 lattice points along X, Y, and Z axes was performed to find
the binding site of complex on the biomacromolecules. Grid point spac-
ing was set to 0.375 Å, to allow the complex to rotate freely. In the next
step, the centre of the grid box was located at the binding site and the
second docking was performed using a cubic box with 60 × 60 × 60 di-
mensions. 250 docking runs with 25,000,000 energy evaluations for all
runs were performed.
measured with excitation wavelength at 295 nm and emission wave-
length range of 300–450 nm. In each measurement, the mixture was
allowed to incubate for 2 min after addition of the compound. Further-
more, all intensities were corrected for the dilution effect in fluores-
cence experiments. Also, for investigation of interaction between PdL
2
compound and DNA, the CT-DNA solution was stirred with EB with
molar ratio of CT-DNA: EB 10:1 and was incubated for 1 h at 4 °C for
completion of interaction between CT-DNA and EB. Then, various
3.2. Molecular dynamics (MD) simulation
amounts of PdL
EB. The fluorescence spectra were measured in the range of
00–700 nm with exciting wavelength at 520 nm. In each measurement
after addition of PdL , the mixture was incubated for 2 min. Moreover,
2
compound were added to the mixture of CT-DNA:
MD simulation method was used to compare the structural
5
changes of HSA and DNA in the absence and presence of PdL
2
com-
2
plex. The PdL -HSA and PdL -DNA complexes with the most negative
2
2
the measured fluorescence intensities were corrected for the dilution
effect (for both of bulk- and nano-scale). Also, the measured fluores-
cence intensities were corrected for the inner-filter effect. To eliminate
the inner filter effects, absorption measurements were carried out at
the fluorescence excitation and emission wavelengths. The extent of
this effect can be roughly evaluated with the following relationship:
free binding energy were considered as the initial conformations for
the MD studies. GROMACS 5.1.2 package [49] and Amber99 force
field [50,51] were used to carry out all MD studies. The partial atomic
2
charges of PdL were calculated using Gaussian 03 [52] at the level of
B3LYP/LanL2dz [53], while the other intramolecular force-field pa-
rameters were generated with ACPYPE [54] and VFFDT [55] for the
organic moiety and the metal center, respectively. Free
[47]
2
biomacromolecules and PdL -biomacromolecule complexes were lo-
Fcorr ¼ Fobs ꢀ eðAex þAemÞ=2
ð1Þ
cated in the cubic box with the periodic boundary conditions in the
three directions. The solutes were placed in the center of box and
the minimum distance between solute surface and the box was
1.0 nm. The box was filled with TIP3P water molecules [56,57], and
the solvated systems were neutralized by adding appropriate
where, Fcorr and Fobs are the corrected and observed fluorescence inten-
sities, respectively, and Aex and Aem are the absorption of PdL at exci-
tation and emission wavelengths, respectively. All the fluorescence
2
+
−
intensities obtained in this study were corrected.
amounts of sodium ions (Na ) and chloride ions (Cl ). After energy
minimization using the steepest descent method, the systems were
equilibrated for 200 ps at the temperature of 300 K. Finally, a 10 ns
MD simulation was carried out at 1 bar and 300 K. Parrinello-
Rahman barostate [58,59] at 1 bar, Berendsen thermostat [60] at
300 K. 9 Å cut off for Van der Walls (VdW) and Coulomb interactions
and the particle mesh Ewald (PME) method [61,62] for long range
electrostatics were used. The leap-frog algorithm with the 2 fs time
step was used to integrate the equation of motions. Finally, an all-
2
.7. Circular dichroism spectroscopy
The circular dichroism (CD) spectra were recorded at room temper-
ature using 0.1 cm quartz cell in the near-UV region (190–260 nm) for
HSA. The CD spectrum of HSA solution was recorded before and after
2
addition of the nano-scale PdL complex with molar ratio 1:1. The anal-
ysis CD of HSA spectra were carried out by CDNN software. The content
of secondary conformation of HSA in each step was calculated from this
technique.
2
bond constrain was used to keep the PdL from drifting and the
atomic coordinates were recorded to the trajectory file every 0.5 ps
for later analysis.
2
.8. Viscosity measurement
3.3. QM/MM study
Viscosity experiments were carried out using a rotational viscometer
and the measurements were performed at 200 rpm at room tempera-
ture. The viscosity of CT-DNA solution was measured in the presence
Own N-layered Integrated Molecular Orbital and molecular Me-
chanics (ONIOM) methodology was employed to carry out QM/MM cal-
culations. The ONIOM can be considered as a hybrid method of
quantum mechanical method (QM) with a molecular mechanics
(MM). This method is able to apply different ab initio or semi-
empirical methods to different parts of a system and combine them to
produce reliable geometry and energy at reduced computational time
of various amounts of the bulk and nano-scale PdL
2
. The obtained data
]/[CT-DNA], where η and η are
viscosity of CT-DNA in the absence and presence of PdL , respectively.
1
/3
are presented as (η/η
0
)
versus [PdL
2
0
2
3. Computational methods
[
63]. In the two layers ONIOM method, the total energy (EONIOM) of
3
.1. Molecular docking calculations
the whole system is a sum of the three independent energy calcula-
tions: [64].
Docking study was carried out to indicate the HSA and DNA-binding
site for PdL complex. The 3D structure of PdL was obtained using the .
cif file of its X-ray crystal structure. The .cif file was converted to the .pdb
format using the Mercury software (http://www.ccdc.cam.ac.uk/). The
crystal structures of HSA (PDB ID: 1AO6) and DNA (PDB ID: 423D)
with sequence d(ACCGACGTCGGT) were taken from the Brookhaven
2
Protein Data Bank (http://www.rcsb.org/pdb). The resolution of these
files was 2.5 and 1.6 Å for HSA and DNA, respectively. Water molecules
2
2
EONIOM2 ¼ Ehigh
low
low
þ E
−E
ð2Þ
model system
real system
model system
Real system contains full geometry of the molecule and is considered
as MM layer while the model system contains the chemically most im-
portant (core) part of the system that is considered as QM layer. Fur-
thermore, the stability of respective adducts was obtained by the

M. Dehkhodaei et al. / Journal of Molecular Liquids 264 (2018) 386–397
389
Scheme 1. A) Synthetic routes for preparation of PdL
spheres of arbitrary radii. C) SEM image of the bulk-scale PdL
2
. B) ORTEP representation of PdL
2
. Displacement ellipsoids are drawn at the 30% probability level and H atoms are shown as small
complex.
2
complex. D) SEM image of the nano-scale PdL
2
evaluation the interaction energy (ΔE), which is calculated by the fol-
lowing equation:
and semi empirical method (PM6) were selected for the low layer and
the high layer, respectively.
ΔE ¼ ΔEBiomacromolecule=PdL2 −ΔEBiomacromolecule−ΔEPdL2
ð3Þ
4. Results and discussion
where ΔEBiomacromolecule/PdL2, ΔEBiomacromolecule and ΔEPdL2 are the energy
4.1. Results of experimental sections
of the optimized adduct of Biomacromolecule-PdL
and PdL , respectively. Biomacromolecule is HSA or DNA here as recep-
tors. A two layers QM/MM method was used for all calculations. HSA
and DNA are considered as low layer while the PdL is considered as
high layer in the current work. Molecular mechanics method (UFF)
2
, Biomacromolecule
2
4.1.1. PdL
The reaction of the Schiff base ligand with metal ion is shown in
Scheme 1A. Also, An ORTEP view of PdL with common atom number-
ing is shown in Scheme 1B. In the solid state, PdL has a crystallographic
2
synthesis and single crystal X-ray diffraction study
2
2
2
2
Fig. 1. The viability percentage of HeLa cancer cells after 48 h exposure to different concentrations of bulk- and nano-scale PdL .

390
M. Dehkhodaei et al. / Journal of Molecular Liquids 264 (2018) 386–397
center of symmetry that is right in the middle point of the palladium (II)
ion in a planar-transoid conformation (Scheme 1B). The molecular unit
is centrosymmetric and is made up of equivalent halves. The structure
Table 1
The DNA and HSA-binding constants (K
b
), the number of binding sites (n) of the PdL
2
.
System
HSA
DNA
of PdL
2
displays a four coordinate palladium atom coordinated to two
(M−1
(M−1
K
b
)
n
K
b
)
n
bidentate N-isopropylsalicylaldimine anions in a slightly distorted
square planer geometry. Each Schiff base ligand acts as a chelate using
a phenolato oxygen and an imino nitrogen. The deprotonated
isopropylsalicylaldimine ligands are arranged with respect to each
other in a trans orientation such that one isopropyl substituent lies
4
4
4
3
Nano-scale PdL
2
1.68 × 10
3.12 × 10
0.93
0.72
5.59 × 10
2.51 × 10
0.95
1.18
Bulk-scale PdL
2
shape and their particles size distribution is about 500–1000 nm.
While, in Scheme 1D the particles have sphere shape and their size dis-
tribution is under 200 nm. As it can be seen in Scheme 1C and D, the syn-
thesized nano particles have more uniform distribution than the
particles in bulk-scale.
above the [PdO
2 2
N ] plane and the other below. The Pd\\O distance
[1.9756(15) Å] and the Pd\\N bond length [2.0368(15) Å] are similar
to those were seen in our previous work [65,66]. For instance, distances
of 1.982(2) Å for Pd\\O bond and 2.017(2) Å for Pd\\N bond are found
in Pd(II) complex with R/S-(1-phenylethylimino)methylnaphtalen-2-ol
ligand [66].
4.1.3. MTT assay
The C_N bond distance is 1.284(3) Å (N1_C7), which is in consis-
tent with a slight elongation of the C_N double bond when coordinates
to a metal center. Close O\\Pd\\N bond angles to 90° [O(1)\\Pd(1)\\N
The results of MTT assay after 48 h exposure of HeLa cancer cells to
different concentrations of bulk and nano-scale PdL
2
were presented
in Fig. 1. Interestingly, the nano-scale PdL displayed potent anticancer
2
(
1) = 88.61(6)°; O(1)\\Pd(1)\\N(1′) = 91.39(6)°] also confirm the
activity as more than 35% cell mortality was obtained after exposure
to 2.4 μM while increasing the concentration to 10 μM led to more
than 53% of cell mortality. In contrast, significant less anticancer activity
distorted square planer geometry for this complex. The crystallographic
data and selected bond lengths and angles are collected in Tables S1–2.
2
was observed in the presence of the bulk-scale PdL as only 29.56% mor-
4
.1.2. Scanning electron microscopy (SEM) measurements
Scheme 1C and D show the morphology of synthesized particles that
tality was obtained in the presence of 10 μM bulk-scale compound and
increasing the concentration to 100 μM was necessary in order to reach
the IC50 level. The results clearly indicated the higher anticancer activity
2
were investigated under SEM for bulk- and nano-scale PdL , respec-
tively. In Scheme 1C the particles exhibit an approximately filament
Fig. 3. A) Fluorescence spectra of the EB-DNA in the absence and presence of increasing
Fig. 2. A) Changes in the fluorescence spectra of HSA in the presence of increasing amounts
amounts of bulk-scale PdL
= 250 μM). B) Plot of Ln((F-F
DNA in the absence and presence of increasing amounts of nano-scale PdL
of [DNA]/[PdL ] = 0–5 ([DNA] = 5 μM, [PdL ] = 25 μM). D) Plot of Ln((F-F
Ln[PdL ].
2
in mol ratio of [DNA]/[PdL
)/F) versus Ln[PdL ]. C) Fluorescence spectra of the EB-
in mol ratio
)/F) versus
2 2
] = 0–5 ([DNA] = 25 μM, [PdL ]
of bulk-scale PdL
B) Plot of Ln((F -F)/F) versus Ln[PdL
presence of increasing amounts of nano-scale PdL
[HSA] = 5 μM, [PdL ] = 25 μM). D) Plot of Ln((F -F)/F) versus Ln[PdL
2
in mol ratio of ([HSA]/[PdL
]. C) Changes in the fluorescence spectra of HSA in the
in mol ratio of ([HSA]/[PdL ] = 0–5
].
2
] = 0–5 ([HSA] = 5 μM, [PdL
2
] = 100 μM).
0
2
0
2
2
2
2
2
2
0
(
2
0
2
2

M. Dehkhodaei et al. / Journal of Molecular Liquids 264 (2018) 386–397
391
of nano-scale PdL
2
than bulk-scale one. It seems that decreasing the size
according to the Stern–Volmer equation [68]:
F0
of the compound significantly affect its biological activity probably due
to its higher bioavailability.
¼
1 þ Ksv½Qꢁ ¼ 1 þ kqτ½Qꢁ
ð5Þ
F
4
.1.4. Fluorescence study for interaction of bulk and nano-scale PdL
2
with
where, F
the absence and presence of the compound, respectively. Ksv is the
Stern-Volmer quenching constant, k is the quenching rate constant of
biomacromolecules and τ is the average lifetime of biomacromolecules
0
and F are the fluorescence intensity of biomacromolecules in
CT-DNA and HSA
The fluorescence spectroscopy has been performed as a reliable and
q
sensitive technique to obtain information about interaction between
bulk- and nano-complex PdL
resent the fluorescence quenching of HSA in the presence of various
amount of bulk- and nano-scale PdL , respectively. Actually, the ob-
2
with CT-DNA and HSA. Fig. 2A and C rep-
without quencher which is typically equal to 10−
biomacromolecules. Ksv is determined from the plot of F /F vs. [Q]
Fig. S1). Fluorescence quenching is classified as two mechanisms: static
8
s
for
0
2
(
served fluorescence emission of HSA is mainly due to the tryptophan
residue. Fig. 2A and C represent that adding the complex in both of
scales, leads to changes of microenvironment around the Trp-214 resi-
due in HSA and causes the fluorescence quenching of HSA. This implies
the appropriate interaction of the compound with HSA as a carrier pro-
tein. The appearance of isosbestic point signifies that equilibrium is
established between HSA and the compound and reflects the formation
of the ground state complex between them. Scachard equation was
used to obtain the binding affinity: [67].
quenching and dynamic quenching. In the static mechanism, the
fluorophore and the quencher collide together in the ground state
while fluorophore and quencher collide together in the excited state
in dynamic mechanism. Linearity of the Stern-Volmer plot indicates
that fluorescence quenching has only one mechanism: dynamic or
q
static. In this case, if the calculated value of k was greater than limiting
diffusion rate constant of the diffusional quenching for biopolymers (2
10
−1 −1
×
10
M
·S ) support that quenching fluorescence occurs by static
mechanism and against that will be dynamic mode.
Here, based on the linearity of Stern-Volmer plot for nano scale
ꢀ
ꢁ
12
−1 −1
F0−F
F
(Fig. S1B) and the obtained value of k
q
(about 10
M
·S ) fluores-
Ln
¼ LnðK Þ þ nLn ½Qꢁ
ð4Þ
b
cence quenching occurs by static mechanism. While, nonlinearity of
Stern-Volmer plot for bulk scale Pd-HSA complex indicates that fluores-
cence quenching is occurred by combination of static and dynamic
modes (see Fig. S1A).
In this equation, F
0
and F are the fluorescence intensity of
On the other hand, investigation of interaction of the compounds with
DNA as the first intracellular target for cancer cells is very important.
Fig. 3A and C represent the fluorescence enhancement of DNA-Ethidium
biomacromolecule in the absence and presence of the compound, re-
spectively. [Q] is the concentration of quencher that quencher is bulk-
and nano-scale PdL
2
, here. “K
b
” is binding constant and is obtained
Bromide (EB) spectrum by adding bulk- and nano-scale PdL
of fluorescence intensity enhancement that F N F , the other type of
Scachard equation was used to obtain binding constant: [69,70].
2
. The case
0
from the plot of Ln((F -F)/F) versus Ln [Q] as y-intercept. These plots
are shown in Fig. 2B and D. “n” is the number of binding site per
biomacromolecule and is determined as slope of the plot. The values
0
2
of n are nearly 1 for PdL in both of scales indicating the compound
binds to HSA with molar ratio of 1:1. The calculated results are pre-
sented in Table 1. To determine the interaction mode of the bulk and
ꢀ
ꢁ
F−F0
F
Ln
¼ LnðK Þ þ nLn ½Qꢁ
ð6Þ
b
2
nano-scale PdL dispersion with HSA, the Stern–Volmer quenching
plots were obtained by monitoring the fluorescence quenching of
0 2
Plots of Ln((F-F )/F) versus Ln[PdL ] for obtaining the binding affin-
biomacromolecules with increasing the concentration of the compound
ity (K ) of the compound to CT-DNA and the number of binding sites on
b
2
Fig. 4. The CD spectra of HSA in the absence and presence of nano-scale PdL complex in the molar ratio of [Complex]/[HSA] = 1 and the results of CDNN software analysis contents of
protein secondary structure.

392
M. Dehkhodaei et al. / Journal of Molecular Liquids 264 (2018) 386–397
Fig. 5. (A) Location of PdL
2
in its interaction with HSA, the amino acids residues of HSA in binding site and its docking clusters. (B) Location of PdL
2
in its interaction with DNA, the base pairs
of DNA in binding site and its docking clusters.
CT-DNA are present in Fig. 3B and D. The results obtained are collected
peaks of α-helical structure of protein attributed to the n-π* transfer
for the bonds of the α-helix [52,53]. According to Fig. 4, the intensity
of HSA peak has been altered slightly due to the binding of nano-
complex. This observation was confirmed by CDNN software analysis
for protein secondary structure contents (Fig. 4).
in Table 1. The values of n for PdL
nearly 1. It means that the compound binds to CT-DNA with molar
ratio of 1:1 (see Table 1). The obtained K values indicate that PdL in
2
on CT-DNA is similar to HSA and is
b
2
its nano-scale interacts stronger than its bulk-scale to CT-DNA. How-
ever, the effect of its size on interaction with HSA is negligible.
To determine the interaction mode of the bulk and nano-scale PdL
dispersion with DNA, the F/F vs. [E] plots were obtained by monitoring
2
4.1.6. Viscosity measurement
0
To further verify the interaction mode of the bulk- and nano-scale
the fluorescence enhancement of biomacromolecules with increasing
the concentration of the compound according to the following equation
PdL
2
with CT-DNA, viscosity measurements of CT-DNA upon addition
were carried out. A classical
of the bulk- and nano-scale PdL
2
[
71]:
intercalative mode causes an increase in the DNA viscosity due to in-
creasing of overall length of DNA [66]. On the other hand, groove bind-
ing and electrostatic interactions could bend the DNA helix, reduce its
length and cause the reduction or no change in the DNA viscosity [66].
F
F0
¼
1 þ KE½Eꢁ
ð7Þ
where, F
0
and F are the fluorescence intensity of biomacromolecules in
Table 2
The average values of RMS deviation (RMSD), number of H-bonds, the radius of gyration
the absence and presence of the compound, respectively. K
hancement rate constant of biomacromolecules. K is determined
from the plot of F/F vs. [E] (Fig. S1).
E
is the en-
(R
g
) and the solvent accessible surface area (SASA) of the HSA, HSA-PdL
2
, DNA and
E
DNA-PdL systems.
2
0
System
HSA
HSA + PdL
DNA
RMSD (nm)
H-bond
Rg (nm)
SASA (nm2)
4
.1.5. Circular dichroism study
The CD spectra of HSA in free and bonded to nano-scale PdL
0.26 ± 0.05
0.25 ± 0.03
0.25 ± 0.10
0.25 ± 0.12
481.5 ± 10.9
470.0 ± 10.9
33.14 ± 1.48
33.17 ± 1.72
2.70 ± 0.02
2.68 ± 0.01
–
–
298.22 ± 3.71
299.00 ± 5.61
2
com-
2
–
–
plex states are shown in Fig. 4. The CD spectrum of HSA indicates two
negative peaks at 208 and 222 nm which are characteristic absorption
DNA + PdL
2

M. Dehkhodaei et al. / Journal of Molecular Liquids 264 (2018) 386–397
393
Fig. 6. (A) Time dependence of RMSD. RMSD values for HSA and HSA-PdL
2
complex during 10 ns MD simulation. (B) Time-evolution of radius of gyration (Rg) for HSA and HSA-PdL
2
complex during 10 ns MD simulation. (C) The number of Intramolecular H-bonds for HSA and HSA-PdL
2
complex during 10 ns MD simulation. (D) RMSF values of HSA and HSA-PdL
2
complex plotted against residue numbers.
2 2
Fig. 7. A) Time dependence of RMSD. RMSD values for DNA and DNA-PdL complex during 10 ns MD simulation. B) RMSF values of DNA and DNA-PdL complex plotted against atom
numbers.

394
M. Dehkhodaei et al. / Journal of Molecular Liquids 264 (2018) 386–397
According to Fig. S2, the viscosity of CT-DNA decreased until molar ratio
of [PdL ]/[DNA] = 0.3 and 1.2 in the presence of bulk- and nano-scale
PdL , respectively and remained constant afterward. The observed
energy which covers H-bonding interactions, Van der Walls,
desolvation and electrostatic energies [39], was estimated as
2
−1
2
−8.63 kcal·mol . The amino acids residues with major role in the sta-
bility of PdL -HSA system are exhibited in Fig. 5A. Also, the docked
model in Fig. 5B for PdL -DNA system suggests that PdL binds in the
minor groove of DNA with ΔG° and intermolecular energy of −5.48
trends confirmed that the binding mode of PdL
2
in its both of scales,
2
may be groove binding.
2
2
−
1
4
4
.2. Results of computational sections
.2.1. Molecular docking studies
and − 6.03 kcal·mol , respectively. These energies are related to
docking conformation (1–1) which has maximum population and min-
imum binding energy. This result is in good agreement with obtained
results of viscosity measurements.
Molecular docking is a virtual powerful tool for drug design. This
computational technique helps researchers to predict and obtain first
insight about stable structure of receptor-ligand complex. In order to
find out the preferred location of the compound on DNA and HSA, mo-
lecular docking studies were carried out. The results of molecular
Fig. 5B represents the base pairs of DNA around of PdL
Also, molecular docking results shows that H-bond or π stacking inter-
actions have not key role in the stability of DNA- or HSA-PdL
and the nature of these interactions are mainly hydrophobic.
2
complex.
2
systems
2
docking for interaction of PdL and HSA indicate that the compound is
situated in the IB subdomain that is known as third drug binding site
of HSA. HSA acts as a carrier for drugs such as Bilirubin, Lidocaine, Indo-
methacin, Hemin and so on. These drugs bind to IB subdomain of HSA
4.2.2. Molecular dynamics simulation
One of the exact computational approaches for investigation of in-
teraction details between biomacromolecules and small molecules is
and transfer in body [72]. Fig. 5A represents the location of PdL
2
and
molecular dynamics (MD) simulation. MD simulation studies for PdL
2
its related cluster during its interaction with HSA. The standard binding
complex with DNA and HSA were carried out and the beginning struc-
tures for the MD analyses were selected from the conformations with
lowest docking energies. The stability of the system (biomacromolecule
and compound) properties was examined by means of RMS deviations
−1
free energy (ΔG°) of this interaction is about −8.08 kcal·mol . This
score is related to the docking cluster with maximum population and
minimum binding energy (see Fig. 5A). Furthermore, intermolecular
Fig. 8. The structure of the PdL
changes of structural parameters of PdL
figure legend, the reader is referred to the web version of this article.)
2
complex and HSA optimized considering charged embedding of MM part in QM part in the gas phase (The blue part of system considers as QM part). The
2
during binding to HSA (Arcs and double arrow show angle and length of bond, respectively). (For interpretation of the references to color in this

M. Dehkhodaei et al. / Journal of Molecular Liquids 264 (2018) 386–397
395
(
RMSD) of unbound and ligand conjugated biomacromolecule with re-
values of DNA and DNA-PdL
2
systems. Fig. 7A indicates that the trajecto-
spect to the initial structure, RMS fluctuations (RMSF), the solvent ac-
cessible surface area (SASA) of biomacromolecule, radius of gyration
ries of both systems are stable and their RMSDs reach equilibrium and
fluctuate around their mean values after about 1 ns simulation time.
Also, local DNA mobility was analyzed by calculating the time averaged
(
Rg) and number of H-bonds. Preliminary simulations over 10 ns time
were performed on DNA and HSA with PdL . Table 2 provides the aver-
age values of RMSD, SASA, Rg and number of H-bonds of the HSA, HSA-
PdL , DNA and DNA-PdL systems. Fig. 6 represents time dependence of
RMSD, Rg, H-bond and RMSF values of HSA and HSA-PdL systems.
2
RMSF values of free DNA and DNA-PdL
against atom numbers based on the last 8000 ps trajectory (Fig. 7B).
The results indicate that the atoms of DNA bound PdL are stable and
have lower RMSF values than the free DNA. Hence, it can be concluded
2
complex and was plotted
2
2
2
2
Fig. 6A indicates that the trajectories of both systems are stable and
their RMSD reached equilibrium and fluctuated around its mean value
after about 5 ns simulation time. Figs. 6B and S3 show that complexation
that the interactions of DNA and PdL
time.
2
were stable during the simulation
with PdL
It can be clearly seen that the Rg was smaller upon binding of the PdL
suggesting a more compact structure of HSA in the presence of PdL . De-
creasing the number of intramolecular H-bonds is another evidence
that confirms a modified structure of HSA in the presence of PdL
2
changed the structure of HSA with compared to free protein.
4.2.3. QM/MM studies
2
In this work a two-layer ONIOM calculations including (PM6: UFF)
were employed to perform QM/MM studies. The molecular mechanics
(MM) was described using the UFF force field for HSA and DNA (low
layer) while semi-empirical quantum mechanics (QM) method (PM6)
2
2
(
Fig. 6C). Also, local protein mobility was analyzed by calculating the
was chosen for the PdL
tained from the molecular docking calculations were opted as starting
points of HSA- and DNA-PdL for the two layer ONIOM studies. All cal-
culations were performed using Gaussian 03 quantum chemistry pack-
age. The partial atomic charges on the atoms of the PdL , DNA and HSA
2
(high layer). The most stable geometries ob-
2
time averaged RMSF values of free HSA and HSA-PdL complex and
was plotted against residue numbers based on the last 5000 ps trajec-
tory (Fig. 6D). The profiles of atomic fluctuations were found to be
very similar to those of free HSA and HSA-PdL
lights the conformational adjustments of the protein structure, in con-
junction with PdL conformational adaptation to its binding sites. The
results indicate that the residues that were in contact with the PdL
are stable and have low RMSF values suggesting that the structure of
drug binding site remains rigid during simulation. Hence, it can be con-
2
2
complex. RMSF high-
2
were considered to reoptimize the optimized geometries. The Van der
Waals and electrostatic interaction were considered in structure optimi-
2
2
zation. Moreover, the polarization of the wave function of the PdL
considered to obtain more truthful geometries. The results of ONIOM
calculations represented that structure of the PdL deviates from its ini-
tial geometry due to binding to HSA and DNA. Along with the interac-
tion of PdL with biomacromolecules some bond lengths and bond
2
was
2
cluded that the interactions of protein and PdL
simulation time. Fig. 7 represents time dependence of RMSD and RMSF
2
were stable during the
2
2 2
Fig. 9. The structure of the PdL complex and DNA optimized considering charged embedding of MM part in QM part in the gas phase. The changes of structural parameters of PdL during
binding to DNA (Arcs and double arrow show angle and length of bond, respectively).

396
M. Dehkhodaei et al. / Journal of Molecular Liquids 264 (2018) 386–397
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the interaction between biomacromolecules and PdL . Figs. 8 and 9
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. Conclusion
[
[
[
Here, synthesis of novel Pd(II) complex in bulk and nano scales using
ultrasonic waves was described. The structure of complex was
established by single crystal X-ray diffraction. The MMT assay results in-
dicated that the anticancer activity of the compound is affected by its
size and notable point is that nano-scale complex reached to IC50 in
1
0 μM of concentration. Investigation of binding affinity, type of interac-
tions and probable changing structure due to interaction of PdL with
2
HSA and DNA, for both of bulk and nano scales complex were carried
out by versatile experimental (Fluorescence spectroscopy, viscosity
measurement, CD spectroscopy) and computational techniques (molec-
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positive effect on the binding affinity. Circular dichroism spectroscopy
was used to investigate the changes on the secondary structure of HSA
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Supplementary data
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2
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